5.3. Types of Observational Studies: Ecologic studies
1. Types of Observational Studies: Ecologic studies
Ecologic studies focus on large groups, rather than on individuals (different from cohorts which are composed of defined individuals). These studies evaluate patterns or trends between outcome and exposure on a geographic, ethnicity, socio-economic status level and can be used to explore potential associations between community-level exposures and disease/outcome. However, ecological studies are unable to reliably estimate individual exposures. On the other hand, details of outcome and exposure can be generalised to the population. The groups can differ by location (for example, city, county, or country). They can also differ by time (a few days, years, or decades).
Ecologic studies may be used to generate hypotheses of an association between exposure and disease for groups, in which members share certain criteria. As other retrospective studies, they can use exiting data and rapidly test the hypothesis. By including groups as a whole a large number of people that can be included in the study and the large number of risk-modifying factors that can be examined. One potential problem with ecologic studies is that just because an association exists between exposure and disease at the group level, it does not mean that a similar association exists at the individual level.
The study by John Snow regarding a cholera outbreak in London is considered the first ecological study to solve a health issue. He used a map of deaths from cholera to determine that the source of the cholera was a pump on Broad Street. He had the pump handle removed in 1854 and people stopped dying there.  It was only when Robert Koch discovered bacteria years later that the mechanism of cholera transmission was understood.
Diet and Alzheimer's
Links between diet and Alzheimer’s disease have been studied using both geographical and temporal ecological studies. The first paper linking diet to risk of Alzheimer’s disease was a multi-country ecological study published in 1997.  It used prevalence of Alzheimer’s disease in 11 countries along with dietary supply factors, finding that total fat and total energy (caloric) supply were strongly correlated with prevalence, while fish and cereals/grains were inversely correlated (i.e., protective). Diet is now considered an important risk-modifying factor for Alzheimer’s disease. 
 Tulchinsky, Theodore H.. “John Snow, Cholera, the Broad Street Pump; Waterborne Diseases Then and Now.” Case Studies in Public Health (2018): 77–99. doi:10.1016/B978-0-12-804571-8.00017-2 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7150208/
 Devanand D, Lee J, Luchsinger J, et al. Lessons from epidemiologic research about risk factors, modifiers, and progression of late onset Alzheimer's Disease in New York City at Columbia University Medical Center. J Alzheimers Dis. 2013;33 Suppl 1(0 1):S447-S455. doi:10.3233/JAD-2012-129041 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4149254/